<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
       input{
           display: inline-block;
           margin-bottom: 15px;
       }
    </style>
</head>
<body>

圆的半径：<input type="text" placeholder="半径" id="a">
<br>
圆的周长：<input type="text" placeholder="周长" id="b">
<br>
元的面积：<input type="text" placeholder="面积" id="c">


<script>
    var p=Math.PI;
    var a=prompt("请输入圆的半径");

    a=parseInt(a);


    document.querySelector("#a").value=a;
    document.querySelector("#b").value=a*2*p;
    document.querySelector("#c").value=a*a*p;


</script>

</body>
</html>